Your program says 6x40m at 90 percent, and you have no idea what that means. So you do what almost every beginner does: you run each rep as hard as you can and call it 90. The problem is that 90 percent and 100 percent feel nearly identical until you have trained yourself to tell them apart. Learning that difference is a skill, and it is one of the highest-leverage things a beginner can learn, because almost every good sprint program is written in percentages you cannot yet feel.
Why programs prescribe percentages at all
Percentages are how a program controls intensity without measuring you every session. A well-designed sprint plan does not want you at 100 percent most of the time. Maximal efforts carry the highest injury risk and the highest nervous-system cost, so a good program spends them carefully, saving true max work for a handful of reps in the right phase. The rest of your training lives in the 80 to 95 percent band, where you can accumulate quality reps without the tax.
That only works if you can actually hit the prescribed number. A rep meant for 90 that you run at 100 is not a smaller version of the right stimulus. It is a different workout, one your program did not schedule, and one your tissues did not agree to. In sprntr the effort percentage is attached to every rep for exactly this reason: the number is the instruction, not a suggestion. But the app cannot run the rep for you. The calibration has to live in your body.
The cues that separate the gears
Effort in sprinting is not one dial. It is several, and they come on at different points. Learning to read them is what lets you find 90 on purpose.
Breathing is the crudest cue and the first one people notice. At 80 percent you finish a 40 with control and could hold a short conversation within a few seconds. At 100 you are gasping and it takes real time to settle. But breathing lags, so it is a poor real-time guide.
Face and jaw tension is more useful, and it is the one most beginners miss. At 95 and above, the jaw clenches, the neck tightens, and the face grimaces. That tension is wasted effort and a reliable sign you have crossed out of the submaximal band. At 90 the face should stay relatively slack. If you catch yourself gritting your teeth, you are not at 90.
Arm reach and rhythm tells you about the top end. As you approach maximum, the arms drive harder and the stride turns over faster, and there is a specific point where the rhythm feels like it is running you rather than you running it. Below that point, at roughly 90, the arms are quick but you are still in charge of the tempo.
The clearest cue of all is the feeling of having another gear. At 90 percent, if someone yelled "go" halfway through, you could find noticeably more speed. That reserve is the whole point. At 100, there is nothing left to find. So the working definition of 90 is simple: fast, smooth, and you can honestly say you had more.
Why running everything at 100 stalls you
Chasing maximum on every rep feels productive and is quietly counterproductive. The first cost is quality. Reps four, five, and six of an all-out session are slower and sloppier than reps one and two, because you are fatigued, and you are teaching your nervous system to produce your sprint pattern under fatigue. That is the opposite of what fast training is for. Submaximal work lets every rep look like your best rep.
The second cost is your hamstrings. Peak eccentric load on the hamstring climbs steeply in the last few percent of speed, so the jump from 90 to 100 is not a 10 percent increase in risk, it is much more. A beginner who runs every rep flat out is spending time in the most dangerous zone on tissue that has not yet adapted to it. Most beginner hamstring pulls happen on a rep the runner did not even intend to make maximal.
The third cost is that you never build a base. Progress in the early weeks comes from volume of good reps, not from a handful of ragged maximal ones. Run everything at 100 and you cannot accumulate that volume, because 100 is too expensive to repeat often. You end up with a few hard sessions, a lot of soreness, and no real foundation.
A drill to calibrate
You cannot learn 90 by reading about it. You learn it by producing efforts on purpose and checking the result. Here is a drill that takes five minutes and teaches the whole scale.
After your warm-up, run three build-up strides over about 50 meters. On the first, aim for 80 percent. On the second, aim for 90. On the third, aim for 95. Do not time them. Instead, right after each one, rate what you actually felt: Was the jaw slack or clenched? Did you have another gear or not? Was the rhythm yours or was it running you?
Most beginners discover their "80" was really 90 and their "90" was really 100. That gap is the thing you are fixing. Do this drill at the front of your sprint days for two or three weeks and the scale starts to feel real. Once it does, the percentages in your program stop being abstract and start being instructions you can follow. That is when a program written in effort actually begins to work.
Where to go next
For how these efforts fit into a week, read how many days per week you should sprint. For why the low-effort days still matter, read what a progressive program is.